The present invention concerns power reserve indicator mechanisms for movements for timepieces of the type fitted with a power source formed of a motor spring. In a conventional manner, the mechanism according to the invention includes a frame, a power reserve indicator and a differential gear with a first input connected to a wheel driven in rotation when the motor spring is wound, a second input connected to a wheel driven in rotation when the motor spring is let down, and an output connected to the power reserve indicator. In this mechanism, the indicator is capable of covering a given angle comprised between two end positions, the first of which is occupied when the motor spring is wound and the second, defined by a stop, when the motor spring is let down.
A mechanism of this type is described in  less than  less than La montre suisse à remontage automatique greater than  greater than  by B. Humbert, Scriptar edition, Lausanne 1955, at page 85. It is provided with an indicator which is friction fitted onto a wheel connected to the output of the differential gear. The indicator moves between two end positions defined by stops. When the indicator reaches one of the stops, of the upper winding or letting down limit of the motor spring, it remains stationary while the motor spring continues to be wound or let down, the friction allowing the gear train to move while the indicator is stationary.
Such a solution thus provides an idea as to the real winding of the motor spring, but there may be significant differences from one cycle to another, due to the relative movement of the indicator with respect to the gear train. The object of the present invention is to overcome this drawback.
The mechanism according to the invention is thus characterized in that an intermediate wheel is inserted between the output of the differential gear and the indicator and includes a resilient member and two coaxial parts, one formed of a wheel, and the other of an arbor, one connected to the differential gear, the other to the indicator and connected to each other by the resilient member, the wheel being arranged so that the two parts rotate together while the indicator occupies a position comprised between the two end positions and in that, at least when the indicator occupies its second end position, the differential gear continues to rotate freely while the indicator remains in abutment and the resilient member is wound, while the motor spring drives the movement.
Advantageously, it is the wheel inserted between the differential gear and the indicator which meshes with the output of the differential gear.
In a particularly advantageous embodiment, the first resilient member includes a cam, secured in rotation to the arbor of the intermediate wheel, and a spring which can deform radially and is rigidly secured by one of its ends to the wheel of the intermediate wheel. This cam cooperates with the free end of the spring. The assembly is arranged such that the free end of the spring slides over the cam when the indicator is in abutment and the motor spring continues to be let down and generate an opposite torque to that exerted by the motor spring.
Consequently, each position of the indictor corresponds to a winding level of the motor spring. Moreover, the timepiece can continue to operate when the indicator has reached the lower limit. The user is, however, informed that the precision of his watch is liable to have been affected by insufficient driving torque, causing a loss of amplitude of the balance.
It is evident that such a mechanism has to include a set of gears to be able to operate normally. This set of gears affects the precision of the displayed information. Also, in order to further improve the agreement between the position of the indicator and the winding of the motor spring, the mechanism according to the invention advantageously includes a second resilient member arranged to exert a permanent torque on the gear train comprised between the two inputs of the differential gear and the intermediate wheel.
In a particularly reliable embodiment, the second resilient member includes an arbor rigidly secured to the frame, a drum provided, at its periphery, with a toothing which meshes with the part of the wheel connected to the indicator, and a strip spring arranged in the drum and secured, by one of its ends, to the arbor of the second resilient member and, by the other, to the drum.
Depending on the way in which the second resilient member works, fluctuations of more or less significance in the amplitude of the balance may result. Indeed, if the motor spring has to wind the second resilient member progressively as it is itself let down, the useful torque decreases to the same extent. The winding torque increases progressively as the motor spring is let down. This is why, in order to assure optimum amplitude of the balance, the second resilient member is arranged such that it is wound and let down simultaneously with the motor spring.
Such a mechanism is particularly well suited to fit a watch including a motor spring assuring a power reserve of 8 days.